1. Filed of the Invention
The present invention relates to a commutator. Such a commutator may be used, for example, in a rotating machine such as an electric motor or generator.
2. Summary of the Prior Art
A commutator is an important component of a rotating machine, and comprises a rotating body with conductive parts (normally segments) which are connected to armature windings of the machine. Brushes pass over the surface of the commutator. Each time a coil of the armature winding is short-circuited by a brush, one brush allows the current to flow from positive to negative polarities and another brush allows current to flow in the reverse direction, from negative to positive polarity. The current flow begins at the instant that the coil is short-circuited by the brush and terminates when the short-circuit is broken. This process is completed in a very short time and repeated continuously to achieve rectification.
The current flowing in a short-circuited coil during the rectification has an increased rate of change when rectification starts or terminates, and a spark is likely to occur between the commutator and brush. Such a rectification spark occurs when the brush contact voltage drop exceeds a specific voltage. The rectification spark is small when there is a low brush contact voltage drop when rectification terminates, and may result in arc discharge when the voltage drop is high. If arc discharge occurs, a high temperature spot may be produced on the brush, so that abnormal abrasion may be caused in and by the brush and the commutator may partially soften. This produces a large number of carbon particles and ionizes air, thereby inducing the danger of flashover.
Additionally, sparks occurring on rectification may result in electric noise regardless of the spark size. This noise induces radio disturbance, causing noise in TV and radio at home (ghost lines or interference voice for TV) and errors in computers.
In order to suppress noise, it has been proposed to connect capacitors between adjacent conductive segments of the commutator, which capacitors inhibit peak currents, and therefore inhibit sparks. There have been many proposals for a configuration of such capacitors, including JP-A-55-15945, JP-A-58-55630, JP-A-59-34056, JP-A-59-34057, JP-A-60-32548, JP-A-55-9197, JP-A-55-9198, JP-A-55-9199, and JP-A-56-3905. In all the above proposals, the capacitor has been a separate component.
It should also be noted that it has been proposed to form a commutator of ceramic material, with the alternate conductive and insulative segments of the capacitor being of different ceramic material. An example of this is JP-A-63-252973 which will be discussed in more detail later. For the sake of completeness it is also mentioned that many different ceramics are known, with different electrical properties, the importance of which will again be discussed later.